High damage threshold frequency conversion system

ABSTRACT

The present invention includes a fundamental laser light source configured to generate fundamental wavelength laser light, a first nonlinear optical crystal configured to generate first alternate wavelength light; a second nonlinear optical crystal configured to generate second alternate wavelength light; a dual wavelength Brewster angle waveplate configured to rotate a polarization of the first alternate wavelength light relative to the second alternate wavelength light such that the first and second alternate wavelength light have the same polarization; a set of Brewster angle wavefront processing optics configured to condition the first and second alternate wavelengths of light; a harmonic separator configured to separate the first alternate wavelength light from the second alternate wavelength light; and a Brewster angle output window configured to transmit the first or second alternate wavelengths of light from the interior of a laser frequency conversion system to the exterior of the laser frequency conversion system.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to and claims the benefit of theearliest available effective filing date(s) from the following listedapplication(s) (the “Related Applications”) (e.g., claims earliestavailable priority dates for other than provisional patent applicationsor claims benefits under 35 USC §119(e) for provisional patentapplications, for any and all parent, grandparent, great-grandparent,etc. applications of the Related Application(s)).

RELATED APPLICATIONS

For purposes of the USPTO extra-statutory requirements, the presentapplication constitutes a regular (non-provisional) patent applicationof United States Provisional Patent Application entitled High DamageThreshold Frequency Conversion System, naming Joseph Armstrong asinventor, filed Nov. 14, 2010, Application Ser. No. 61/413,469.

TECHNICAL FIELD

The present invention generally relates to a frequency converted lightsource suitable for implementation in an illuminator of a semiconductorwafer or photomask inspection system, and more particularly, to afrequency conversion system having a high damage threshold.

BACKGROUND

As the dimensions of semiconductor devices and components continue todecrease, the demand for semiconductor wafer and photomask inspectionsystems exhibiting high throughput and improvements in resolutioncontinue to increase. One manner in which higher levels of resolutionare attained in semiconductor and photomask inspections systems includesthe utilization of an illumination source capable of emitting shorterwavelength light.

Certain practical advantages may be achieved when illuminating a waferor reticle with light having wavelengths at or below 400 nm. Providingsuitable lasers for high quality wafer and photomask inspection systemspresents a particular challenge. Conventional lasers capable ofgenerating deep ultraviolet (DUV) light energy are typically large,expensive, and suffer from relatively short lifetimes and low averagepower output. In order to obtain adequate throughput and defectsignal-to-noise ratio (SNR), wafer and photomask inspection systemsgenerally require a laser based illumination source having high averagepower, low peak power, and relatively short wavelength.

Conventionally, the primary method for providing adequate DUV powerentails converting long wavelength light to shorter wavelength light,referred to herein as “frequency conversion.” It is well known in theart that frequency conversion from longer wavelength light to shorterwavelength is often accomplished utilizing one or more non-linearoptical crystals. In this context, frequency conversion requires highpeak power light in order to produce a nonlinear response in a givennon-linear optical crystal. To increase the efficiency of this processthe longer wavelength light may be generated to have high average power,short optical pulses, and may be focused into the optical crystal. Theoriginal “longer wavelength” light is commonly referred to as“fundamental light.”

Generating light at wavelengths below 400 nm, and especially below 300nm, is challenging. Light sources implemented in semiconductorinspection systems require relatively high powers, long lifetimes, andstable performance. Light sources meeting these requirements foradvanced inspection techniques are nonexistent in the prior art. Thelifetime, power, and stability of current DUV frequency converted lasersare generally limited by the implemented frequency conversion crystaland frequency conversion scheme. This is particularly true fornon-linear conversion crystals exposed to DUV wavelengths, such as, butnot limited to, 355, 266, 213, and 193 nm.

Many inspection applications require the frequency converted laser poweror wavefront to remain stable over time. Due to degradation of theoptical coatings, as a result of exposure to high power illumination,maintaining power and wavefront stability over time is challenging. Thisis especially true for optical coatings in the UV-DUV portion of thegiven frequency conversion system. These optical elements are typicallynot shifted so they must survive for the lifetime of the laser,typically greater than 10,000 hours and even 20,000 hours. Mirrors inthe DUV below 350 nm are typically limited to power densities ofapproximately 100 W/cm², and even lower for wavelengths less than 250nm. This constraint forces optical components such as lenses and mirrorsto be placed far away from the frequency conversion crystal in order toreduce the power density on the optical coatings. In the case of UVlasers with power levels greater than 0.5 W this requirement may lead toan unrealistically larger laser system.

Accordingly, it is therefore desirable to have optics in the UV-DUVportion of a frequency conversion system that can withstand very highpower densities without changing over time. It is also desirable thatthese optics be efficient for a given wavelength range of interest,producing a minimum amount of stray light. Meeting these requirementsmay extend the lifetime of an implementing laser, reduce operating costsand laser maintenance time, and increase overall laser reliability.

SUMMARY

An apparatus for laser frequency conversion having high damage thresholdis disclosed. In one aspect, the apparatus may include, but is notlimited to, a fundamental laser light source configured to generatefundamental wavelength laser light; a first nonlinear optical crystalconfigured to receive fundamental laser light from the fundamental laserlight source, the first optical crystal configured to generate firstalternate wavelength light by frequency converting at least a portion ofthe received fundamental laser light to first alternate wavelengthlight; a second nonlinear optical crystal configured to receive firstalternate wavelength light from the first nonlinear optical crystal, thesecond nonlinear optical crystal configured to generate second alternatewavelength light by frequency converting at least a portion of thereceived first alternate wavelength light to second alternate wavelengthlight; a dual wavelength Brewster angle waveplate configured to receivefirst alternate wavelength light and second alternate wavelength lightemanating from the second nonlinear optical crystal, the dual wavelengthBrewster angle waveplate further configured to rotate a polarization ofthe first alternate wavelength light relative to the second alternatewavelength light such that the first alternate wavelength light and thesecond alternate wavelength light have substantially the samepolarization; a set of Brewster angle wavefront processing opticsconfigured receive first alternate wavelength light and second alternatewavelength light from the dual wavelength Brewster angle waveplate, theset of Brewster angle wavefront processing optics further configured tocondition the first alternate wavelength light and second alternatewavelength light emanating from the dual wavelength Brewster anglewaveplate; a harmonic separator configured to receive the firstalternate wavelength light and the second alternate wavelength lightfrom the set of Brewster angle wavefront processing optics, the harmonicseparator configured to at least partially separate the first alternatewavelength light from the second alternate wavelength light; and atleast one Brewster angle output window configured to transmit at leastone of the first alternate wavelength light or the second alternatewavelength light from an interior portion of a laser frequencyconversion system to a region external to the laser frequency conversionsystem.

In another aspect, an apparatus may include, but is not limited to, afundamental laser light source configured to generate fundamentalwavelength laser light; at least one nonlinear optical crystalconfigured to generate alternate wavelength light by frequencyconverting at least a portion of received laser light to alternatewavelength light; a dual wavelength Brewster angle waveplate configuredto receive fundamental wavelength light and alternate wavelength lightemanating from the at least one nonlinear optical crystal, the dualwavelength Brewster angle waveplate further configured to rotate apolarization of the fundamental wavelength light relative to thealternate wavelength light such that the fundamental wavelength lightand the alternate wavelength light have substantially the samepolarization; a set of Brewster angle wavefront processing opticsconfigured to receive fundamental wavelength light and alternatewavelength light from the dual wavelength Brewster angle waveplate, theset of Brewster angle wavefront processing optics further configured tocondition the fundamental wavelength light and the alternate wavelengthlight emanating from the dual wavelength Brewster angle waveplate; aharmonic separator configured to receive fundamental wavelength lightand alternate wavelength light from the set of Brewster angle wavefrontprocessing optics, the harmonic separator configured to at leastpartially separate the fundamental wavelength light from the alternatewavelength light; and at least one Brewster angle output windowconfigured to transmit at least one of the fundamental wavelength lightor the alternate wavelength light from an interior portion of a laserfrequency conversion system to a region external to the laser frequencyconversion system.

A method for laser frequency conversion with high damage threshold isdisclosed. In one aspect, the method may include, but is not limited to,providing a fundamental laser light source; generating fundamentalwavelength laser light utilizing the fundamental laser light source;generating first alternate wavelength light by frequency converting atleast a portion of the fundamental laser light to first alternatewavelength light utilizing a first nonlinear optical crystal; generatingsecond alternate wavelength light by frequency converting at least aportion of the first alternate wavelength light to second alternatewavelength light utilizing a second nonlinear optical crystal; rotatinga polarization of the first alternate wavelength light relative to thesecond alternate wavelength light such that polarization of the firstalternate wavelength light and polarization of the second alternatewavelength light are substantially the same utilizing a dual wavelengthBrewster angle waveplate; conditioning the first alternate wavelengthlight and second alternate wavelength light utilizing Brewster anglewavefront processing optics; separating at least a portion of the firstalternate wavelength light from the second alternate wavelength lightutilizing a harmonic separator including a harmonic separation elementhaving a first Brewster angle surface, a total internal reflectionsurface, and a second Brewster angle surface; and transmitting at leastone of the first alternate wavelength light or the second alternatewavelength light from an interior portion of a laser frequencyconversion system to a region external to the laser frequency conversionsystem.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not necessarily restrictive of the invention as claimed. Theaccompanying drawings, which are incorporated in and constitute a partof the specification, illustrate embodiments of the invention andtogether with the general description, serve to explain the principlesof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the disclosure may be better understood bythose skilled in the art by reference to the accompanying figures inwhich:

FIG. 1 illustrates a block diagram of an apparatus for laser frequencyconversion with a high damage threshold, in accordance with anembodiment of the present invention.

FIG. 2 illustrates a schematic diagram of an apparatus for laserfrequency conversion with a high damage threshold, in accordance with anembodiment of the present invention.

FIGS. 3A-3B illustrate a schematic diagram of a Brewster angle dualwavelength waveplate of the apparatus for laser frequency conversionwith a high damage threshold, in accordance with an embodiment of thepresent invention.

FIGS. 4A-4B illustrate a schematic diagram of a Brewster angle dualwavelength waveplate of the apparatus for laser frequency conversionwith a high damage threshold, in accordance with an embodiment of thepresent invention.

FIG. 5 illustrates a schematic diagram of a set of Brewster anglewavefront processing optics of the apparatus for laser frequencyconversion with a high damage threshold, in accordance with anembodiment of the present invention.

FIG. 6 illustrates a schematic diagram of a set of Brewster anglewavefront processing optics of the apparatus for laser frequencyconversion with a high damage threshold, in accordance with anembodiment of the present invention.

FIG. 7 illustrates a schematic diagram of a harmonic separator of theapparatus for laser frequency conversion with a high damage threshold,in accordance with an embodiment of the present invention.

FIG. 8 illustrates a schematic diagram of a harmonic separator of theapparatus for laser frequency conversion with a high damage threshold,in accordance with an embodiment of the present invention.

FIG. 9 illustrates a schematic diagram of a harmonic separator of theapparatus for laser frequency conversion with a high damage threshold,in accordance with an embodiment of the present invention.

FIG. 10 illustrates a schematic diagram of a harmonic separator of theapparatus for laser frequency conversion with a high damage threshold,in accordance with an embodiment of the present invention.

FIG. 11 illustrates a schematic diagram of a dual Brewster angle outputwindow of the apparatus for laser frequency conversion with a highdamage threshold, in accordance with an embodiment of the presentinvention.

FIG. 12 illustrates a flow diagram of a method for laser frequencyconversion with high damage threshold, in accordance with an embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the subject matter disclosed,which is illustrated in the accompanying drawings.

Referring generally to FIGS. 1 through 11, a frequency conversion systemwith high damage threshold is described in accordance with the presentinvention.

The present disclosure is directed to a system for laser frequencyconversion capable of withstanding high power illumination at smallwavelengths (e.g., below 400 nm). In order to produce high beam powersat wavelengths less than 400 nm, fundamental light sources combined withmultiple stages of frequency conversion are typically used. Frequencyconversion of continuous or quasi-continuous light often requires thelight to be focused to very small spots within a utilized nonlinearcrystal. Optics that are in proximity to the frequency conversioncrystal, especially optics in regions where light at wavelengths of lessthan 400 nm is created, may suffer significant degradation to opticalcoatings used for anti-reflection, reflection, and harmonic separation.Typical damage thresholds for lifetimes of 10,000 hours are 30-100watts/cm² and may be much less for wavelengths below 250 nm. Aconservatively configured laser producing 1 W of power in a 1 mmdiameter beam corresponds to a power density of 127 W/cm². In addition,high peak powers produced by pulsed lasers may further shorten coatinglifetimes. In contrast, uncoated optics often have surface damagethresholds that approach that of the bulk material. In the UV-DUV range,this is typically many orders of magnitude greater than opticalcoatings.

It is an object of the present invention to provide a frequencyconverted laser system which operates in the absence of opticalcoatings. The frequency converted laser system of the present inventionincludes various novel optical approaches, including, but not limitedto, a Brewster angle dual wavelength waveplate, Brewster angle lenses,and Brewster angle-total internal reflection (TIR) harmonic separationelements. It is further contemplated that in addition to improved damagethreshold, the implementation of the various optical elements describedthrough the present disclosure may also provide: reduced stray lightwith the system, 100% theoretical efficiency for the desired wavelength,and reduced system size due to the dramatic increase in damagethreshold. Moreover, since no optical coatings are utilized, the systemmay easily be modified to work with any desired wavelength capable oftransmission through glass or crystalline material.

FIG. 1 illustrates a block diagram view of a frequency conversion system100 with high damage threshold, in accordance with an embodiment of thepresent invention. In one aspect, the system 100 may include afundamental laser light source 102 configured to generate laser light103 of a fundamental wavelength (e.g., wavelength of ω), a firstnon-linear optical crystal 104 configured to receive the fundamentallaser light 103 and frequency convert at least a portion of thefundamental light 103 to a first alternate wavelength light 105 (e.g.,light having a wavelength of 2ω), and a second nonlinear optical crystal110 configured to receive the first alternate wavelength light 105generated by the first optical crystal 104 and frequency convert atleast a portion of the first alternate wavelength light 105 to secondalternate wavelength light 111 (e.g., light having a wavelength of 4ω).In a further aspect, the system 100 may include a set of dichroicharmonic separation optics 106 configured to separate the fundamentallight 103 and the first alternate light 105 emanating from the firstnonlinear optical crystal 104. In another aspect, the system 100 mayinclude a Brewster angle dual wavelength waveplate 112 configured torotate the polarization of the first alternate wavelength light 105relative to the second alternate wavelength light 111 such that,following rotation, the first alternate wavelength light 113 and thesecond alternate wavelength light 115 have substantially the samepolarization. In an additional aspect, the system 100 may include a setof Brewster angle wavefront processing optics 114 configured tocondition (e.g., focus, collimate, correct, or the like) the firstwavelength light 113 and the second wavelength light 115 followingprocessing by the dual wavelength waveplate 112. In another aspect, thesystem 100 may include a set of harmonic separation optics 116configured to separate the first alternate wavelength light 117 (e.g.,2ω) from the second alternate wavelength light 119 (e.g., 4ω) followingemanation from the wavefront processing optics 114. Applicant notes thatthroughout the present disclosure the term “harmonic separation optics”is used interchangeably with “harmonic separator.” Further, the system100 may include one or more Brewster angle output windows 118 configuredto transmit the second alternate wavelength light 121 from the interiorof the system 100 to a selected target 120.

In a general sense, the fundamental laser light source 102 may includeany laser light source known in the art. The particular choice ofwavelength of the fundamental laser light source 102 may depend on avariety of factors, including, but not limited to, the desired outputwavelength of the alternate wavelength light generated by the non-linearresponse of the first non-linear optical crystal 104 or the secondnon-linear optical crystal 110.

Further, the first and second non-linear optical crystals 104, 110 ofthe system 100 may include any non-linear optical crystal known in theart. The particular choice of the non-linear optical crystal may dependon a variety of factors, including, but not limited to, the desiredoutput wavelength of the alternate wavelength light generated by thenon-linear response of the optical crystals 104, 110. In this sense, thechoice of the fundamental laser light source 102 and the opticalcrystals 104 and 110 are typically made in conjunction, wherein aparticular combination of the fundamental laser light source 102 and thefirst non-linear optical crystal 104 and the second non-linear opticalcrystal 110 are chosen such that the set produce the desired outputalternate wavelength light 111. The particular choice of the fundamentallight source 102, the optical crystals 104 and 110, or the firstgenerated alternate wavelength light 105 or the second generatedalternate wavelength light 111 are not limiting and it should berecognized by those skilled in the art that any choice of the above arewithin the scope of the present invention.

In one embodiment of the present invention, the dichroic separationoptics 106 configured to separate the fundamental wavelength light 103from the first alternate wavelength light 105 emanating from the firstnonlinear optical crystal 104 may include, but are not limited to, adichroic mirror. In this manner, the dichroic mirror may be arranged totransmit the fundamental light 103 to a beam dump 108, while directingthe first alternate light 105 to the second nonlinear optical crystal110. It should be recognized, however, that this description is notlimiting and numerous other frequency separating technologies may beutilized within the scope of the present invention. While it isrecognized herein that uncoated optics may be used to separate thefundamental wavelength light 103 from the first alternate wavelengthlight 105, it is anticipated that the first alternate wavelength light105 (e.g., second harmonic light) is likely to be greater than 400 nm.At wavelengths of this size, optical coatings typically provide suitabledamage thresholds.

While the configuration depicted in FIG. 1 is suitable for secondharmonic generation, it is recognized herein that other types offrequency conversion or frequency mixing process may be implementedwithin the scope of the present invention.

The present invention is further directed to a system suitable forseparating the divergent and copropagating first alternate wavelengthbeam 105 and the second alternate wavelength beam 111 after they emergefrom the second nonlinear optical crystal 110 in order to isolate theselected frequency converted beam (e.g., beam having frequency of 4ω).In addition to isolating the high order beam, the system 100 maycondition the illumination emanating from the second nonlinear opticalcrystal 110 by focusing, collimating, and/or correcting (e.g.,correcting astigmatism) the beam of second alternate light 115 (e.g.,beam having frequency of 4ω).

It recognized herein that, upon exiting the second nonlinear opticalcrystal 110, the second alternate light (e.g., 4ω) is often orthogonallypolarized with respect to the first alternate light (e.g., 2ω). In ageneral sense, it is recognized herein that Brewster angle surfaces arehighly efficient to P-polarized light, while being inefficient forS-polarized light. In this sense, a typical S-polarized reflection atthe Brewster angle is only 16% of the original incident light. As such,the utilization of multiple Brewster angle reflections allows for the100% efficient transmission of P-polarized second alternate wavelengthlight 111 (e.g., 4ω), with multiple stray reflections occurring forS-polarized first alternate light 105 (e.g., 2ω). It is recognized thatthis behavior may lead to problems related to laser control, as theinconsistency is problematic with respect to laser control loops andfeedback control. Applicant notes that the above description related tothe polarization of the first alternate wavelength light 105 and thesecond alternate wavelength light 111 is not limiting and should merelybe interpreted as an illustration. For example, it is anticipated thatprior to processing by the waveplate 112, the second alternatewavelength light beam 111 may have polarization states other than pureP-polarization, while the first alternate wavelength light beam 105 mayexhibit polarization states other than pure S-polarization.

In one aspect of the invention, the Brewster angle dual wavelengthwaveplate 112 of the system 100 may effectively rotate the polarizationof the first alternate wavelength light 105 relative to the secondalternate wavelength light 111 such that, following rotation of thepolarization, the first alternate wavelength light 113 and the secondalternate wavelength light 115 have substantially the same polarization.For example, in the case where the first nonlinear optical crystal 104and the second nonlinear optical crystal 110 are both arranged in asecond harmonic generation configuration, the Brewster angle dualwavelength waveplate 112 may effectively rotate the polarization of 2ωlight transmitted through the second optical crystal 110 relative to the4ω light generated by the second optical crystal 110, such that the 2ωand 4ω light emanating from the second optical crystal 110 have the samepolarization. Further, the Brewster angle surfaces implementedthroughout the present invention may be selected such that they arehighly efficient to both wavelengths.

It is recognized herein that during processing by the Brewster angledual wavelength waveplate 112 the polarization of the first alternatewavelength light 105 (2ω) is not rotated at the first surface of thedual wavelength waveplate 112. As such, there will be a single 16%reflection at the impinging surface of the dual wavelength waveplate forS-polarized first alternate wavelength light 105 (2ω). It is furtherrecognized that the polarization of the first alternate wavelength light105 (2ω) is rotated by the time it reaches the subsequent surfaces ofthe dual wavelength waveplate 112. As such, there is minimal lossobserved at the subsequent surfaces of the dual wavelength waveplate112. After processing by the dual wavelength waveplate 112, the exitingfirst alternate wavelength light 113 (2ω) has a polarization which isthe same as the polarization of the exiting second alternate wavelengthlight 115 (4ω).

In another aspect of the invention, the Brewster angle wavefrontprocessing optics 114 may condition the wavefront of the beam emergingfrom the dual wavelength waveplate 112, consisting of first alternatewavelength light 113 (2ω) and the second alternate wavelength light 115(4ω). For example, the Brewster angle wavefront processing optics 114may include a set of lenses oriented at the Brewster Angle andconfigured to collimate, correct (e.g., correct astigmatism), or focusthe wavefront of the beam emerging from the waveplate 112. It iscontemplated herein that the lenses of the set of Brewster anglewavefront processing optics 114 may be used on axis or at small anglesfrom the axis due to the large amount of astigmatism produced by tiltingone or more lenses, as shown in greater detail further herein. It isrecognized, however, that an off axis cylindrical lens element may beimplemented in order to correct for all or a portion of the astigmatism.As such, the implemented wavefront processing optics may both collimateand correct for the astigmatism typically produced in the frequencyconversion process (e.g., frequency conversion process taking place insecond non-linear optical crystal 110). It is further recognized thatthe Brewster's angle lens is likely only practical for small variationsin the angle of incidence around Brewster's angle. Large variations inthe angle of incidence may produce additional intensity roll off in theprocessed beam. It is further contemplated that additional Brewster'sangle lenses may also be implemented.

In an additional aspect of the present invention, the harmonicseparation optics 116 (i.e., the harmonic separator) may separate thefirst alternate wavelength light 117 (2ω) from the second alternatewavelength light 119 (4ω) emanating from the wavefront processing optics114. The harmonic separation optics 116 may include a Brewster Angleinput surface, a total internal reflection (TIR) surface, and a BrewsterAngle output surface. As will be discussed in greater detail furtherherein, it is recognized that the harmonic separation optics 116 of thepresent invention may include various configurations. It is furthercontemplated herein that the TIR surface of the harmonic separationoptics 116 may be engineered to produce TIR for the second alternatewavelength light 119 (4ω), while transmitting the first alternatewavelength light 117 (2ω).

In another aspect of the present invention, after harmonic separation,the second alternate wavelength light beam 121 (4ω) may be transmittedthrough a Brewster angle output window 118. For example, the secondalternate wavelength light beam 121 (4ω) may be transmitted through oneor more Brewster angle output windows 118 toward a selected target 120.In addition, the first alternate wavelength light beam 123 (2ω) may beutilized in another capacity (e.g., directed at a target required firstalternate wavelength light) or transmitted to a beam dump 108.

The applicant notes that since the entire optical path for the secondalternate wavelength light (4ω) utilizes Brewster angle surfaces and TIRsurfaces, approximately 100% transmission of the second alternatewavelength light (4ω) is attainable.

While the above description focuses on the implementation of the system100 with a first crystal 104 and a second crystal 110 and each of theprocessing elements including the Brewster angle dual wavelengthwaveplate 112, the Brewster angle wavefront processing optics 114, andthe harmonic separation optics 116, it is further contemplated hereinthat the system 100 does not require each of these components. Forexample, it is contemplated herein that the system 100 may include asingle nonlinear optical crystal, with the output of the singlenonlinear optical crystal being fed into the various Brewster angleprocessing components described previously herein. Further, it iscontemplated herein that the system 100 may include one or more of theprocessing components including the Brewster angle dual wavelengthwaveplate 112, the Brewster angle wavefront processing optics 114, andthe harmonic separation optics 116. As such, the description throughoutthe present disclosure relating to specific configurations of thefrequency conversion system of the present invention should not beinterpreted as limiting, but merely illustrative.

FIG. 2 illustrates a schematic view of a single nonlinear opticalcrystal 208 based frequency conversion system 200, in accordance withone embodiment of the present invention. In one aspect, the system 200includes a fundamental light source 202, a focusing element 206, asingle nonlinear optical crystal 208, a Brewster angle dual wavelengthwaveplate 214, Brewster angle wavefront processing optics 216, andharmonic separation optics 218.

For the purposes of the present disclosure it is assumed that the singlenonlinear crystal 208 is configured to receive horizontally polarized204 fundamental laser light 201 having a frequency of 532 nm from thefundamental laser light source 202. Further, the optical crystal 208 isconfigured to frequency convert at least a portion of the receivedhorizontally polarized 532 nm light to vertically polarized 210alternate wavelength light 212 having a 266 nm wavelength. Applicantnotes that the above description related to the frequency andpolarization of the fundamental light 201 or the alternate wavelengthlight 212 is not limiting and should be interpreted merely as anillustration. Further, it is also contemplated herein that the system200 may incorporate the concepts discussed with respect to system 100.Likewise, it is further contemplated that the following description ofsystem 200 and the various components thereof should be interpreted toextend to system 100. For instance, although not shown, rather than asingle nonlinear optical crystal the system 200 may include a firstnonlinear optical crystal configured to generate first alternatewavelength light and a second nonlinear optical crystal configured togenerate second alternate wavelength light. As such, the descriptionrelated to system 100 should be interpreted to extend to system 200.

In one aspect, the focusing element 206 is disposed along an opticalpathway between the fundamental light source 202 and the nonlinearoptical crystal 208. In this regard, the focusing element 206 isconfigured to focus the fundamental light 201 emanating from thefundamental light source 202 into the nonlinear optical crystal 208.

It is noted herein that the arrangement described above relating to theposition of the focusing element 206 is not limiting and should merelybe interpreted as illustrative in nature. In a further embodiment, thefocusing element 206 may include any optical devices known in the artsuitable for focusing the fundamental laser light 201 emanating from thefundamental laser light source 202. For example, the first focusingelement 206 may include, but is not limited to, a lens, a mirror, or adiffractive element.

It is contemplated herein that focusing of the fundamental laser light201 is not a requirement of the system 200. In this sense, whetherfocusing of the fundamental laser light 201 is required may depend, forexample, on the required beam size within the optical crystal 208.

The focusing element 206 may focus horizontally polarized 532 nm lightthrough the nonlinear crystal 208 in order to generate 266 nm verticallypolarized light. After frequency conversion inside the nonlinear opticalcrystal 208 the alternate wavelength light 212 (266 nm) having avertical polarization 210 copropagates with residual fundamental light201 (532 nm) having a horizontal polarization 204 toward the Brewsterangle dual wavelength waveplate 214.

In another aspect of the present invention, upon emanating from thenonlinear optical crystal 208, the alternate wavelength light 212 (266nm) and the residual fundamental light 201 (532 nm) is processed by theBrewster angle dual wavelength waveplate 214. In a general sense, thoseskilled in the art should recognize that a dual wavelength waveplate iscapable of providing a specific retardance at two different wavelengths.The dual wavelength waveplate 214 of the present invention may includeany known dual wavelength waveplate known in the art. For example, thedual wavelength waveplate 214 may include, but is not limited to, acrystal quartz based dual wavelength waveplate.

In one embodiment, the Brewster angle dual wavelength waveplate 214 mayinclude a first surface at S-polarization for the fundamental light (532nm) and at P-polarization for the alternate wavelength light (266 nm).As such, the surface reflects approximately 16% of the fundamental light201 impinging on the first waveplate surface, while transmittingapproximately 100% of the alternate wavelength light 212. In a furtherembodiment, the dual wavelength waveplate 214 may be constructed toprovide full wave retardation at the alternate light wavelength (266nm), while providing half wave retardation at the fundamental lightwavelength (532). As such, the waveplate 214 does not rotate thepolarization of the alternate light 212, while the waveplate 214 doesrotate the polarization of the fundamental light 204, so that thefundamental light (532 nm) is also at Brewster's angle and there isminimal reflection at the second surface of the waveplate 214.

FIGS. 3A and 3B illustrate a schematic view of a Brewster angle dualwavelength waveplate 214, in accordance with one embodiment of thepresent invention. The dual wavelength waveplate 214 of FIG. 3A mayinclude, but is not limited to, a single birefringent plate 312. Thoseskilled in the art should recognize that commercially availablewaveplates typically are used at normal incidence, with the optical axisof the crystal within the surface of the waveplate. In this sense,commercial waveplates are typically used to rotate polarizations tovarious angles by rotating polarization about the axis normal to thewaveplate surface. It is further recognized that multiple order dualwavelength waveplates may be implemented, however, because of theassociated material dispersion, the performance at one of thewavelengths is typically not optimum. Further, the utilization ofwaveplates at normal incidence is common because the normal incidencegeometry simplifies the manufacturing process as the optical axisresides within the surface of the plate, dramatically simplifying theeffort required to solve the required crystal orientation.

In the case of Brewster angle incidence, there exist three primaryclasses of solutions to the required orientation. The three classes ofsolutions include: 1) a single plate with the crystal axis out of theplate surface; 2) a single plate with the crystal axis in the platesurface; and 3) dual plates with the crystal axis within the platesurface.

As shown in FIG. 3A, the waveplate 214 of FIG. 3A consists of a singleplate 312 having a crystal axis out of the plate surface. InputS-polarized fundamental light 304 (e.g., wavelength of 532 nm) andP-polarized alternate light 306 (e.g., wavelength of 266 nm) may impingeon a surface of the waveplate 214 at Brewster's angle. For example, inthe case of 266 nm light and a crystal quartz waveplate, the Brewsterangle is 58 degrees. It is noted herein that the implemented angle ofincidence is approximately equal to Brewster's angle, which depends onthe index of refraction of the receiving waveplate, which in turn isdependent on the wavelength of the incident light beam. As such, theimplemented angle of incidence for the dual wavelength waveplate is afunction of the material utilized in the plate as well as the wavelengthof the light impinging on the waveplate.

It is noted that a portion of the impinging fundamental light 304 (532nm) light will reflect 314 off a first surface 313 of the single plate312. It is further noted that the crystal axis 316 is rotated at anangle θ with respect to the waveplate surface 313 and at an angle φ withrespect to the polarization axis 324 of the alternate wavelength light(266 nm), as shown by 322 of FIG. 3B.

In one embodiment, the thickness of the plate 312 and the angle of thecrystal axis may be chosen such that the alternate wavelength light 306(266 nm) is retarded by exactly one full wave and the fundamental light204 (532 nm) is within 99% of half wave retardation. In thisorientation, the alternate wavelength light (266 nm) experiences nochange in propagation through the single waveplate 312, while thefundamental wavelength light 204 (532 nm) experiences a polarizationrotation of 90 degrees. It is recognized herein that because the singlewaveplate 312 is oriented at Brewster's angle for the alternatewavelength light 212 (266 nm) there is no theoretical loss at thiswavelength, the fundamental wavelength light 204 (532 nm), however, asit has S-polarization for the first surface of the waveplate 312,experiences approximately 16% reflection at the first surface 313.

In another embodiment, the plate 312 may be configured to provide fullwave retardation at the fundamental wavelength and half wave retardationat the alternate wavelength. In an alternative embodiment, the plate 312may be configured to provide half wave retardation at both thefundamental wavelength and the alternate wavelength.

It is further noted that once the alternate wavelength light 212 (532nm) reaches the second surface 315 of the single plate 312, thepolarization is rotated to P-polarization. The reflectivity offundamental light 204 (532 nm) P-polarized light at an interface that isat Brewster's angle for the alternate wavelength light 212 (266 nm) isonly approximately 0.05%, resulting in very little loss. As such, thesingle birefringent plate 312 of FIG. 3 is suitable for effectivelyrotating the fundamental laser light 204 (532 nm) polarization to thesame polarization of the alternate wavelength light 212 (266 nm) withonly a single, but manageable, reflection of the fundamental light 201.

It is noted that while the above description focuses on fundamentallight and alternate light having 532 nm and 266 nm wavelengthsrespectively, it is recognized herein that the concepts above may beextended to other incident wavelengths and orientation angles.

FIGS. 4A and 4B illustrate a schematic view of a Brewster angle dualwavelength waveplate 214, in accordance with one embodiment of thepresent invention. The dual wavelength waveplate 214 of FIG. 4A includesa differential waveplate 214 consisting of a first plate 412 a and asecond plate 412 b optically coupled to the first plate 412 a. Further,the second plate 412 b may act to correct at least a portion of thephase variations introduced by the first plate 412 a.

Those skilled in the art should recognize the difficulty that exists intrying to manufacture a free standing waveplate that is less than 300 μmin thickness. Waveplates that are thicker than this size often displayincreased sensitivity angle, which is problematic when the givenwaveplate must be utilized over a range of angles. The waveplate 214design of FIG. 4A is less sensitive to variations in the angle ofincidence and further allows for increased plate thickness, whichfurther simplifies the manufacturing process.

The first plate 412 a and the second plate 412 b of the dual wavelengthwaveplate 214 may be coupled together in manner known in the art. In oneembodiment, the first plate 412 a and second plate 412 b may beoptically coupled such that an air gap exists at the interface 415between the two plates. In another embodiment, the first plate 412 a andsecond plate 412 b may be coupled together utilizing an epoxy. In yetanother embodiment, the first plate 412 a and second plate 412 b mayoptically contacted or bonded together.

FIG. 4A illustrates a dual wavelength Brewster angle waveplate 214having a first plate 412 a and the second plate 412 b optically bondedtogether. In one aspect, the crystal axis 416 of the first plate 412 aand the crystal axis 418 of the second plate 412 b are in the planes ofthe respective plates, as shown in FIG. 4A. As previously discussed,impinging alternate wavelength light 2212 (266 nm) is P-polarized, whileimpinging fundamental wavelength light 201 (532 nm) is S-polarized.

In another aspect, the waveplates 412 a and 412 b are set at Brewster'sangle for 266 nm light. At the first surface 413 of the dual wavelengthwaveplate 214, approximately 16% of the fundamental light 201 (532 nm)is reflected 211, while the alternate wavelength light 212 (266 nm)experiences zero or nearly-zero reflection at the first surface 413 andis allowed to efficiently pass through the waveplate 214.

In one embodiment, the thickness of the first waveplate 412 a may beselected such that it behaves as a full waveplate at the alternatewavelength (266 nm) and a half waveplate at the fundamental wavelength(532 nm). This configuration minimizes any reflection at the interface415 between first waveplate 412 a and the second waveplate 412 b of thedual wavelength waveplate 214. In another embodiment, the thickness ofthe second waveplate 412 b may be chosen such that the difference inthickness is equivalent to the thinnest available single waveplatesolution for the dual wavelength Brewster angle waveplate 214.

In an additional embodiment, the crystal axis 416 of the first waveplate412 a is orientated at an angle φ1 with respect to the polarization axis426 of the alternate wavelength light 212 (266 nm), while the crystalaxis 418 of the second waveplate 412 b is oriented at an angle φ2 withrespect to the polarization axis 426 of the alternate wavelength light212 (266 nm). It is recognized herein that when φ1 is equal and oppositeto φ2 the second waveplate 412 b acts to cancel out at least a portionof the retardance of the first waveplate 412 a, resulting in adifferential dual wavelength Brewster angle waveplate. It is furtherrecognized herein that the differential solution is less sensitive tochanges in angle of incidence, temperature, alignment, and the like.

The birefringent plates of the dual wavelength waveplate 214 may befabricated from any suitable material known in the art. For example,material used to fabricated the one or more birefringent plates of thedual wavelength waveplate 214 may include crystal quartz, magnesiumfluoride, sapphire lithium niobate, or a rutile based compound.

Referring again to FIG. 2, upon emanating from the Brewster angle dualwavelength waveplate 214, the vertically polarized alternate wavelengthlight 212 (266 nm) and the vertically polarized residual fundamentallight 201 (532 nm) is further condition utilizing a set of Brewsterangle wavefront processing optics 216. The wavefront processing optics216 of the present invention may include focusing optics, collimatingoptics, or correction optics. In one embodiment, the lens or set of lensof the wavefront processing optics 216 may be configured to collimate,focus, and/or correct the alternate light 212 (266 nm) after it emanatesfrom the dual wavelength waveplate 214 It is recognized herein that thisconfiguration may produce residual aberrations that may be correctedonce the beams are separated. Applicant notes herein that the opticalelements of the Brewster angle wavefront processing optics 216 mayinclude any optical devices known in the art capable of carrying out thecorrection, collimation, or focusing as required by the system 200.

FIG. 5 illustrates a schematic view of a set of Brewster angle wavefrontprocessing optics 216, in accordance with an embodiment of the presentinvention. As shown in the embodiment of FIG. 5, the Brewster anglewavefront processing optics 216 may include, but are not limited to, afirst lens 504 a and a second lens 504 b. It is recognized herein thatthe lens or set of lenses (e.g., lenses 504 a and 504 b) of theprocessing optics 216 may be utilized at normal incidence or at smallangles in order to minimize aberrations within the system. It is furtherrecognized that titling of one or more of lenses used to process lightmay cause astigmatism in the processed light. In addition, decenteringof one or more of the lenses may produce coma in the processed light.The Brewster angle wavefront processing optics 216, among other things,are configured to minimize the effects of aberration, astigmatism, andcoma utilizing one or more surfaces oriented at the Brewster angle.

It is recognized herein that in order for light 502 (e.g., fundamentallight and/or alternate light) to experience low reflections over theentire beam the angles of incidence should extend over a small rangeabout the Brewster angle. In order to meet this requirement processingoptics 216 may include lenses having a long radius of curvature, whilethe incident light may include light having a high F number (e.g., Fnumber of 50 or higher). It is anticipated that the use of a F numberequal to or above 50 along with a lens design having a suitably longradius of curvature may maintain the required low angle of incidence.

In one embodiment, illustrated in FIG. 5, the wavefront processingoptics 216 may include a fully corrected two lens system with allsurfaces being at the Brewster's angle. In one embodiment, the firstlens 504 a of the wavefront processing optics 216 may have a firstsurface that has a spherical curvature and a second surface that isplanar. Further, the second lens 504 b of the processing optics 216 mayhave a first surface that is planar and second surface that hascylindrical curvature. The cylindrical curvature of the second lens 504b acts to correct for the astigmatism of the tilted spherical surface ofthe first lens 504 a and aids in producing an output beam 508 with nonet astigmatism. It is further noted that the output beam 508 isdisplaced relative to the input beam 502 as it propagates through bothlenses 504 a and 504 b. In a further embodiment, the lens system may beengineered such that the center of the input beam 502 passes through thepoint on the curved surface of the first lens 504 a where the anglebetween the surface normal of the lens 504 a and the beam propagationaxis are at Brewster's angle.

In another embodiment, the first and second lenses 504 a and 504 b ofthe wavefront processing optics 216 may be configured to produce aselected focal length. For example, the lens design of the wavefrontprocessing optics 216 of FIG. 5 may include a lens design with a focallength of 155 mm.

It is further recognized herein that the lens design of the wavefrontprocessing optics 216 may be selected to produce varying degrees ofresidual astigmatism. This ability may then be used to correct forastigmatism that typically exists in frequency converted lasers.

FIG. 6 illustrates a schematic view of a set of Brewster angle wavefrontprocessing optics 216, in accordance with an alternative embodiment ofthe present invention. In this embodiment, the first lens 604 a may beoriented at Brewster's angle, with the second lens 604 b may be orientedat the Brewster angle, but in a compensating direction, as shown in FIG.6. In this sense, the cylindrical surface of the second lens 604 b isoriented at an angle of 90 degrees relative to the cylindrical surfaceof the second lens 504 b of the embodiment illustrated in FIG. 5. It isrecognized herein that if both lenses 604 a and 604 b are made with thesame center thickness, the output beam 608 should experience no netdisplacement relative to the input beam 602. It is further recognizedherein that the configuration illustrated in FIG. 6 may be implementedin a frequency conversion system in order to simplify system alignment.

In another embodiment, the wavefront processing optics 216 may include alens system configured to correct for astigmatism by adding astigmatismwith no net focusing power. For example, an input wavefront having ameasured level of astigmatism may be corrected using two elements eachhaving a cylindrical surface such that output wavefront has zeroresidual astigmatism. For instance, an input wavefront with 3.3278 wavesof astigmatism may be corrected using two elements each having acylindrical surface such that the residual astigmatism of the outputbeam is zero to within numerical error.

In another embodiment, the amount of astigmatism present in a given beammay be dynamically adjusted by changing the axial separation of thefirst lens (e.g., 504 a or 604 a) and the second lens (e.g., 504 b or604 b). It is recognized herein that the dynamic adjustment ofastigmatism allows for fine tuning the astigmatism correction duringinitial alignment and correcting for changing astigmatism as thefrequency converted laser ages. It is further contemplated that thisdynamic astigmatism correct may be carried out utilizing an astigmatismmeasurement system, a computer system, and one or more translationstages communicatively coupled in a feedback loop.

Referring again to FIG. 2, upon emanating from the Brewster anglewavefront processing optics 216, the alternate wavelength light 212 (266nm) and the residual fundamental light 201 (532 nm) may be transmittedthrough one or more Harmonic Separation Optical elements 218. Applicantagain notes that throughout the present disclosure the term “harmonicseparation optical elements” is used interchangeably with “harmonicseparator.” The harmonic separation optics 218 may include one or moreBrewster Angle-TIR harmonic separation elements. It is recognized hereinthat a variety of harmonic separation configurations may be implementedwithin the present invention, as will be described in greater detailfurther herein. For example, it is recognized that the one or moreharmonic separation optical elements 218 may include a single harmonicseparation element or multiple harmonic separation elements.

For instance, the harmonic separator 218 may include a first harmonicseparation element 220 and a second harmonic separation element 222. Itis recognized herein that the second harmonic separation element 222 maybe implemented in order to further increase the angular separation ofthe harmonics (i.e., the fundamental light beam and the alternatewavelength light beam) allowed for the reduction in size of the givenfrequency conversion system. In another instance, a second harmonicseparation element may be implemented in order to redirect the beamindependent of the harmonic separation.

FIG. 7 illustrates a Brewster angle-Total internal Reflection (TIR)reflector element 700 suitable for implementation as a harmonicseparation optical element of the harmonic separator 218 of the presentinvention. It is recognized herein that ideally the light impinging onthe harmonic separation element 700 is highly P-polarized, improving theefficiency of the element 700. It is noted, however, that this is not arequirement of the invention.

In one embodiment, a first optical surface 704 of the element 700receives light at Brewster's angle θ_(B). The light is then transmittedthrough a portion of the element 700 and undergoes TIR on a secondsurface 706 of the element 700. The light then exits through a thirdsurface 708 of the element 700 oriented at Brewsters angle θ_(B).

It is recognized herein that if two wavelengths (e.g., 532 nm forfundamental wavelength light and 266 nm for alternate wavelength light)enter the harmonic separation element 700, the light of each wavelengthwill undergo different angles of refraction causing the beams to deviatefrom one another through the volume of the element 700. In this example,the second Brewster's angle surface may add an opposite angle to theangular deviation resulting in zero net angular change once the beamsexit the element 700. It is noted that there may exist a residualspatial offset between the two beams, which is dependent on the lengthof propagation in within the element 700. In a further embodiment, thisspatial offset may be corrected for utilizing a second BrewsterAngle-TIR element if desired.

In another embodiment, the angle 712 between the incident light 702 andthe exiting light 710 may be controlled by using different apex angles.The minimum deviation may be set by the requirement for TIR at thesecond surface 706.

It is recognized herein that the material of the harmonic separationelement 700 may include any suitable material known in the art. Forexample, the material of the harmonic separation element 700 mayinclude, but is not limited to, any glass or crystalline material. It isfurther recognized that in the UV-DUV range fused silica and calciumfluoride are particular advantageous.

Those skilled in the art should recognize that light beams of shorterwavelength light bend more significantly when entering a material with ahigher index of refraction than the surrounding medium (e.g., air). Assuch, the harmonic separation element 700 may be configured such thatthe shorter wavelength light undergoes TIR, while longer wavelength islargely transmitted through the element 700. In this regard, the element700 acts to separate the incident wavelengths of light.

In another embodiment, in the event only a single wavelength is incidenton the element 700, the element 700 may be utilized as a zero loss, highdamage threshold mirror.

FIG. 8 illustrates an alternative Brewster angle-Total internalReflection (TIR) reflector element 800 suitable for implementation as aharmonic separation optical element 218 of the present invention.Applicant notes that for the purposes of the present disclosure thedescription related to FIG. 7 should be interpreted to extend throughoutthe remainder of the disclosure unless otherwise noted.

In one embodiment, a first optical surface 804 is configured to receivelight at Brewster's angle θ_(B). The light is then transmitted through aportion of the element 800 and undergoes TIR on a second surface 806 ofthe element 800. At least a portion of the light then exits through athird surface 814 of the element 800 oriented at Brewster's angle θ_(B).It is noted herein that the second Brewster's angle surface 814 of theBrewster angle-TIR reflector element 800 is oriented in the negative oropposite direction as the second Brewster's angle surface 708 of theelement 700 of FIG. 7.

It is again noted herein that if two or more wavelengths (e.g., 532 nmfor fundamental wavelength light and 266 nm for alternate wavelengthlight) enter the harmonic separation element 800, the light of eachwavelength will undergo different angles of refraction at the firstBrewster surface 804 causing the beams 810, 812 to begin deviating fromone another. In this example, the second Brewster's angle surface 814may add an additional dispersion angle to the angular deviation. As thetwo wavelengths propagate after exiting the second Brewster anglesurface 814 of the element 800 they will continue to increase theirseparation, until they are completely separated, allowing for theisolation of the two or more wavelengths of light.

In another embodiment, the angle 816 between the incident light 802 andthe exiting light 810, 812 may be controlled by using different apexangles. The minimum deviation may be set by the requirement for TIR atthe second surface 806.

It is again recognized that the material of the harmonic separationelement 800 may include any suitable material known in the art. Forexample, the material of the harmonic separation element 800 mayinclude, but is not limited to, any glass or crystalline material. It isfurther recognized that in the UV-DUV range fused silica and calciumfluoride are particular advantageous.

Due to shorter wavelength light bending more significantly when enteringa material with a higher index of refraction, the harmonic separationelement 800 may be configured such that the shorter wavelength lightundergoes TIR at surface 806, while longer wavelength light (e.g., light808) is largely transmitted through the through the element 800 and outof the surface 806. In this regard, the element 800 also acts toseparate the incident wavelengths of light.

FIG. 9 illustrates a combination 900 of a harmonic separation element901 and a Brewster angle element 915, wherein the Brewster angle element915 is implemented to increase the net angular deviation between lightbeams of two wavelengths emerging from the harmonic separation element901. As noted previously, it is recognized herein that ideally the lightimpinging on the first harmonic separation element 901 is highlyP-polarized, improving the efficiency of the elements 901 and 915. It isnoted, however, that this is not a requirement of the invention.

In one embodiment, a first optical surface 904 is configured to receivelight at Brewster's angle θ_(B). The light is then transmitted through aportion of the element 901 and undergoes TIR on a second surface 906 ofthe element 901. At least a portion of the light then exits through athird surface 914 of the element 901 oriented at Brewster's angle θ_(B).

It is again noted that if two or more wavelengths (e.g., 532 nm forfundamental wavelength light and 266 nm for alternate wavelength light)enter the harmonic separation element 901, the light of each wavelengthwill undergo different angles of refraction at the first Brewstersurface 904 causing the beams 910, 912 to begin deviating from oneanother. In this example, the second Brewster's angle surface 914 mayadd an additional dispersion angle to the angular deviation. As the twowavelengths propagate after exiting the second Brewster angle surface914 of the element 901 they will continue to increase their separation,until they impinge on the Brewster angle element 915.

In one embodiment, the input optical surfaces 916 and 918 of theBrewster angle element 915 may be oriented at Brewster's angle for thedifferent impinging wavelengths (e.g., 532 nm for fundamental wavelengthlight and 266 nm for alternate wavelength light). It is recognizedherein that since these Brewster surfaces 916, 918 have different signs,the difference in the refraction angles increases greatly. In a furtherembodiment, the output surface may be optimized at Brewster's angle forone of the wavelengths, typically the wavelength of interest (i.e., thealternate wavelength light). For example, an antireflection coating maybe added to one section of the element 915 to further reduce thereflectivity.

The Brewster angle element 915 may be fabricated from any suitablematerial known in the art. For example, the material of the Brewsterangle element 915 may include, but is not limited to, any glass orcrystalline material, such as, but not limited to, fused silica andcalcium fluoride.

FIG. 10 illustrates an alternative Brewster Angle-TIR element 1000suitable for implementation as a harmonic separation optical element 218of the present invention. In one aspect, the element 1000 may includeone or more curved surfaces (curvature not shown) in order to providefocus the incident beam 1002. In a further embodiment, the curvedsurface of the element 1000 may include the TIR surface 1006 of theelement 1000, allowing the element to focus light reflected from the TIRsurface 1006.

It is recognized herein that the combination of the off axis use of afocusing surface may cause astigmatism. In a further embodiment, theelement 1000 may include curved surfaces at the Brewster angle surfaces1004 and/or 1008 in order to correct for the astigmatism create by thecurved TIR surface or produce a desired amount of astigmatism in theoutput beam 1010. In another embodiment, external Brewster angleelements (not shown) may also be utilized to fully or partiallycompensate for the astigmatism produced by the curved TIR 1006 surface.For example, the external Brewster angle elements used to correct theastigmatism may include one or more curved spherical or cylindricalBrewster angle surfaces.

FIG. 11 illustrates a dual Brewster angle window output 1100 suitablefor implementation in a frequency conversion laser system, in accordancewith one embodiment of the present invention. In one embodiment, thewindows 1110 are oriented at Brewster's angle for the verticallypolarized 11044 light 1102 exiting the enclosure 1104 of the frequencyconversion system (e.g., 200 or 100), which acts to reduce or eliminateloss. It is noted herein that two windows may be utilized in order toallow for the rotation or replacement of the external window withoutexposing the internal portion of the laser to photocontamination. Inanother embodiment, the output window 1100 may include a purge system(not shown) configured to maintain laminar flow 1116 of a purging gasacross the external portion of the outside window in order to limitphotocontamination to this window. It is recognized herein that anysuitable gas known in the art may be implemented to provide laminar flowacross the external surface of the outside window of the window output1100. For example, the purge gas may include an inert gas, such asnitrogen or argon.

In another embodiment, the Brewster angle window output 1100 may beconfigured to allow an emerging beam 1103 (e.g., fundamental wavelengthlight or alternate wavelength light) to be transmitted off-axis, thatis, along a direction oriented at a non-zero angle relative to theoptical axis of the optical system of the laser frequency conversionsystem.

In another embodiment, the windows 1110 of the dual Brewster anglewindow output 1100 may be fabricated utilizing any material known in theart suitable for orienting the windows at Brewster's angle for theexiting light 1102. For example, the windows 1110 may include, but arenot limited to, fused silica or calcium based windows. It is recognizedthat the above window materials do not represent limitations, but aremerely illustrative.

In an additional embodiment, the each of the windows 1110 may be sealedto the laser enclosure 1106 utilizing a seal 1112. For example, the seal1112 may include, a low-outgassing o-ring material, such as VITON orKALRES. In another example, the seal 1112 may include a metal sealconstructed from a soft metal ring, such as a metal gasket.

FIG. 12 illustrates a process flow diagram of a method 1200 for laserfrequency conversion with high damage threshold, in accordance with oneembodiment of the present invention. At step 1202, the method 1200provides a fundamental laser light source. At step 1204, the method 1200generates fundamental wavelength laser light utilizing the fundamentallaser light source. At step 1206, the method 1200 generates firstalternate wavelength light by frequency converting a portion of thefundamental laser light to first alternate wavelength light utilizing afirst nonlinear optical crystal. At step 1208, the method 1200 generatessecond alternate wavelength light by frequency converting a portion ofthe first alternate wavelength light to second alternate wavelengthlight utilizing a second nonlinear optical crystal. At step 1210,utilizing a dual wavelength Brewster angle waveplate, the method 1200rotates the polarization of the first alternate wavelength lightrelative to the second alternate wavelength light such that polarizationof the first alternate wavelength light and polarization of the secondalternate wavelength light are substantially the same. At step 1212, themethod 1200 conditions (e.g., focusing, collimating, correctingastigmatism, correcting aberration) the first alternate wavelength lightand the second alternate wavelength light utilizing Brewster anglewavefront processing optics. At step 1214, the method 1200 separates atleast a portion of the first alternate wavelength light from the secondalternate wavelength light utilizing a harmonic separator, the harmonicseparator including a harmonic separation element having a firstBrewster angle surface, a total internal reflection surface, and asecond Brewster angle surface. At step 1216, the method 1200 transmitsthe first alternate wavelength light or the second alternate wavelengthlight from an interior portion of the laser frequency conversion system(e.g., 100 or 200) to a region external to the laser frequencyconversion system.

Those having skill in the art will appreciate that there are variousvehicles by which processes and/or systems and/or other technologiesdescribed herein can be effected (e.g., hardware, software, and/orfirmware), and that the preferred vehicle will vary with the context inwhich the processes and/or systems and/or other technologies aredeployed. For example, if an implementer determines that speed andaccuracy are paramount, the implementer may opt for a mainly hardwareand/or firmware vehicle; alternatively, if flexibility is paramount, theimplementer may opt for a mainly software implementation; or, yet againalternatively, the implementer may opt for some combination of hardware,software, and/or firmware. Hence, there are several possible vehicles bywhich the processes and/or devices and/or other technologies describedherein may be effected, none of which is inherently superior to theother in that any vehicle to be utilized is a choice dependent upon thecontext in which the vehicle will be deployed and the specific concerns(e.g., speed, flexibility, or predictability) of the implementer, any ofwhich may vary. Those skilled in the art will recognize that opticalaspects of implementations will typically employ optically-orientedhardware, software, and or firmware.

Those skilled in the art will recognize that it is common within the artto describe devices and/or processes in the fashion set forth herein,and thereafter use engineering practices to integrate such describeddevices and/or processes into data processing systems. That is, at leasta portion of the devices and/or processes described herein can beintegrated into a data processing system via a reasonable amount ofexperimentation. Those having skill in the art will recognize that atypical data processing system generally includes one or more of asystem unit housing, a video display device, a memory such as volatileand non-volatile memory, processors such as microprocessors and digitalsignal processors, computational entities such as operating systems,drivers, graphical user interfaces, and applications programs, one ormore interaction devices, such as a touch pad or screen, and/or controlsystems including feedback loops and control motors (e.g., feedback forsensing position and/or velocity; control motors for moving and/oradjusting components and/or quantities). A typical data processingsystem may be implemented utilizing any suitable commercially availablecomponents, such as those typically found in datacomputing/communication and/or network computing/communication systems.

The herein described subject matter sometimes illustrates differentcomponents contained within, or connected with, different othercomponents. It is to be understood that such depicted architectures aremerely exemplary, and that in fact many other architectures can beimplemented which achieve the same functionality. In a conceptual sense,any arrangement of components to achieve the same functionality iseffectively “associated” such that the desired functionality isachieved. Hence, any two components herein combined to achieve aparticular functionality can be seen as “associated with” each othersuch that the desired functionality is achieved, irrespective ofarchitectures or intermedial components. Likewise, any two components soassociated can also be viewed as being “connected”, or “coupled”, toeach other to achieve the desired functionality, and any two componentscapable of being so associated can also be viewed as being “couplable”,to each other to achieve the desired functionality. Specific examples ofcouplable include but are not limited to physically mateable and/orphysically interacting components and/or wirelessly interactable and/orwirelessly interacting components and/or logically interacting and/orlogically interactable components.

While particular aspects of the present subject matter described hereinhave been shown and described, it will be apparent to those skilled inthe art that, based upon the teachings herein, changes and modificationsmay be made without departing from the subject matter described hereinand its broader aspects and, therefore, the appended claims are toencompass within their scope all such changes and modifications as arewithin the true spirit and scope of the subject matter described herein.

Although particular embodiments of this invention have been illustrated,it is apparent that various modifications and embodiments of theinvention may be made by those skilled in the art without departing fromthe scope and spirit of the foregoing disclosure. Accordingly, the scopeof the invention should be limited only by the claims appended hereto.

It is believed that the present disclosure and many of its attendantadvantages will be understood by the foregoing description, and it willbe apparent that various changes may be made in the form, constructionand arrangement of the components without departing from the disclosedsubject matter or without sacrificing all of its material advantages.The form described is merely explanatory, and it is the intention of thefollowing claims to encompass and include such changes.

Furthermore, it is to be understood that the invention is defined by theappended claims.

What is claimed:
 1. An apparatus for laser frequency conversion having ahigh damage threshold, comprising: a fundamental laser light sourceconfigured to generate fundamental wavelength laser light; a firstnonlinear optical crystal configured to receive fundamental laser lightfrom the fundamental laser light source, the first nonlinear opticalcrystal configured to generate first alternate wavelength light byfrequency converting at least a portion of the received fundamentallaser light to first alternate wavelength light; a second nonlinearoptical crystal configured to receive first alternate wavelength lightfrom the first nonlinear optical crystal, the second nonlinear opticalcrystal configured to generate second alternate wavelength light byfrequency converting at least a portion of the received first alternatewavelength light to second alternate wavelength light; a dual wavelengthBrewster angle waveplate configured to receive first alternatewavelength light and second alternate wavelength light emanating fromthe second nonlinear optical crystal, the dual wavelength Brewster anglewaveplate further configured to rotate a polarization of the firstalternate wavelength light relative to the second alternate wavelengthlight such that the first alternate wavelength light and the secondalternate wavelength light have substantially the same polarization; aset of Brewster angle wavefront processing optics configured receivefirst alternate wavelength light and second alternate wavelength lightfrom the dual wavelength Brewster angle waveplate, the set of Brewsterangle wavefront processing optics further configured to condition thefirst alternate wavelength light and second alternate wavelength lightemanating from the dual wavelength Brewster angle waveplate; a harmonicseparator configured to receive the first alternate wavelength light andthe second alternate wavelength light from the set of Brewster anglewavefront processing optics, the harmonic separator configured to atleast partially separate the first alternate wavelength light from thesecond alternate wavelength light; and at least one Brewster angleoutput window configured to transmit at least one of the first alternatewavelength light or the second alternate wavelength light from aninterior portion of a laser frequency conversion system to a regionexternal to the laser frequency conversion system.
 2. The apparatus ofclaim 1, wherein the first nonlinear optical crystal is configured forsecond harmonic generation.
 3. The apparatus of claim 1, wherein thesecond nonlinear optical crystal is configured for second harmonicgeneration.
 4. The apparatus of claim 1, further comprising: anadditional nonlinear optical crystal configured to receive secondalternate wavelength light from the second nonlinear optical crystal,the additional nonlinear optical crystal configured to generate analternate wavelength light by frequency converting at least a portion ofthe received second alternate wavelength light to an additionalalternate wavelength light.
 5. The apparatus of claim 1, furthercomprising: one or more dichroic harmonic separation elements configuredto separate the fundamental light from the first alternate light, theone or more dichroic harmonic separation elements configured to directat least a portion of the first alternate light to the second nonlinearoptical crystal.
 6. The apparatus of claim 1, wherein the dualwavelength Brewster angle waveplate comprises: a birefringent plate. 7.The apparatus of claim 1, wherein the dual wavelength Brewster anglewaveplate comprises: a first birefringent plate and a secondbirefringent plate.
 8. The apparatus of claim 1, wherein the dualwavelength Brewster angle waveplate includes at least one birefringentplate comprising at least one of a crystal quartz birefringent plate, amagnesium fluoride birefringent plate, a sapphire birefringent plate, alithium niobate birefringent plate, or a rutile birefringent plate. 9.The apparatus of claim 1, wherein the dual wavelength Brewster anglewaveplate comprises: a waveplate having an optical axis within a planeof the waveplate.
 10. The apparatus of claim 1, wherein the dualwavelength Brewster angle waveplate comprises: a waveplate having anoptical axis outside of a plane of the waveplate.
 11. The apparatus ofclaim 1, wherein the dual wavelength Brewster angle waveplate is a halfwave retarder for the first alternate wavelength light and a full waveretarder for the second alternate wavelength light.
 12. The apparatus ofclaim 1, wherein the dual wavelength Brewster angle waveplate is a fullwave retarder for the first alternate wavelength light and a half waveretarder for the second alternate wavelength light.
 13. The apparatus ofclaim 1, wherein the dual wavelength Brewster angle waveplate is a halfwave retarder for the first alternate wavelength light and a half waveretarder for the second alternate wavelength light.
 14. The apparatus ofclaim 1, wherein the set of Brewster angle wavefront processing opticscomprises: at least one lens.
 15. The apparatus of claim 14, wherein theat least one lens comprises: at least one fused silica lens or at leastone calcium fluoride lens.
 16. The apparatus of claim 1, wherein the setof Brewster angle wavefront processing optics comprises: a first lensand a second lens.
 17. The apparatus of claim 16, wherein a surface ofthe first lens or a surface of the second lens comprises a cylindricalsurface.
 18. The apparatus of claim 16, wherein the set of Brewsterangle wavefront processing optics are configured to adjust theseparation between the first lens and the second lens in order to adjustfor astigmatism in at least one of the first alternate wavelength lightor second alternate wavelength light emanating from the dual wavelengthBrewster angle waveplate.
 19. The apparatus of claim 1, wherein the setof Brewster angle wavefront processing optics are configured to correctfor astigmatism in at least one of the first alternate wavelength lightor the second alternate wavelength light emanating from the dualwavelength Brewster angle waveplate.
 20. The apparatus of claim 1,wherein the set of Brewster angle wavefront processing optics areconfigured to focus at least one of the first alternate wavelength lightor the second alternate wavelength light emanating from the dualwavelength Brewster angle waveplate.
 21. The apparatus of claim 1,wherein the set of Brewster angle wavefront processing optics areconfigured to collimate at least one of the first alternate wavelengthlight or the second alternate wavelength light emanating from the dualwavelength Brewster angle waveplate.
 22. The apparatus of claim 1,wherein the set of Brewster angle wavefront processing optics areconfigured to correct abberative effects in at least one of the firstalternate wavelength light or the second alternate wavelength lightemanating from the dual wavelength Brewster angle waveplate.
 23. Theapparatus of claim 1, wherein the harmonic separator comprises: aharmonic separation element, the harmonic separation element having afirst Brewster angle surface, a total internal reflection (TIR) surface,and a second Brewster angle surface, the TIR surface between the firstBrewster angle surface and the second Brewster angle surface.
 24. Theapparatus of claim 23, wherein the TIR surface comprises: a curved TIRsurface, wherein the curved TIR surface is configured to focus lightreflected from the TIR surface.
 25. The apparatus of claim 23, whereinat least one of the first Brewster angle surface or the second Brewsterangle surface is curved in order to compensate for aberrations producedby a curvature of the TIR surface.
 26. The apparatus of claim 23,further comprising: an additional Brewster angle element configured tocompensate for aberrations produced by a curvature of the TIR surface.27. The apparatus of claim 23, wherein the at least one lens comprises:at least one fused silica lens or at least one calcium fluoride lens.28. The apparatus of claim 1, wherein the harmonic separator comprises:a first harmonic separation element and a second harmonic separationelement.
 29. The apparatus of claim 1, wherein the harmonic separatorcomprises: a harmonic separation element and a Brewster angle element,the Brewster angle element configured to receive a first wavelength oflight and a second wavelength of light from the first harmonicseparation element.
 30. The apparatus of claim 1, wherein the at leastone Brewster angle output window comprises: a first Brewster angleoutput window and a second Brewster angle output window, the firstBrewster angle output window arranged between the interior of the laserfrequency conversion system and the second Brewster angle output window,the second Brewster output window arranged between the region externalto the laser frequency conversion system and the first Brewster angleoutput window.
 31. The apparatus of claim 30, further comprising: apurge system configured to provide laminar flow of a gas across an outersurface of the second Brewster angle output window.
 32. An apparatus forlaser frequency conversion having a high damage threshold, comprising: afundamental laser light source configured to generate fundamentalwavelength laser light; at least one nonlinear optical crystalconfigured to generate alternate wavelength light by frequencyconverting at least a portion of received laser light to alternatewavelength light; a dual wavelength Brewster angle waveplate configuredto receive fundamental wavelength light and alternate wavelength lightemanating from the at least one nonlinear optical crystal, the dualwavelength Brewster angle waveplate further configured to rotate apolarization of the fundamental wavelength light relative to thealternate wavelength light such that the fundamental wavelength lightand the alternate wavelength light have substantially the samepolarization; a set of Brewster angle wavefront processing opticsconfigured to receive fundamental wavelength light and alternatewavelength light from the dual wavelength Brewster angle waveplate, theset of Brewster angle wavefront processing optics further configured tocondition the fundamental wavelength light and the alternate wavelengthlight emanating from the dual wavelength Brewster angle waveplate; aharmonic separator configured to receive fundamental wavelength lightand alternate wavelength light from the set of Brewster angle wavefrontprocessing optics, the harmonic separator configured to at leastpartially separate the fundamental wavelength light from the alternatewavelength light; and at least one Brewster angle output windowconfigured to transmit at least one of the fundamental wavelength lightor the alternate wavelength light from an interior portion of a laserfrequency conversion system to a region external to the laser frequencyconversion system.
 33. A method for laser frequency conversion with highdamage threshold, comprising: providing a fundamental laser lightsource; generating fundamental wavelength laser light utilizing thefundamental laser light source; generating first alternate wavelengthlight by frequency converting at least a portion of the fundamentallaser light to first alternate wavelength light utilizing a firstnonlinear optical crystal; generating second alternate wavelength lightby frequency converting at least a portion of the first alternatewavelength light to second alternate wavelength light utilizing a secondnonlinear optical crystal; rotating a polarization of the firstalternate wavelength light relative to the second alternate wavelengthlight such that polarization of the first alternate wavelength light andpolarization of the second alternate wavelength light are substantiallythe same utilizing a dual wavelength Brewster angle waveplate;conditioning the first alternate wavelength light and second alternatewavelength light utilizing Brewster angle wavefront processing optics;separating at least a portion of the first alternate wavelength lightfrom the second alternate wavelength light utilizing a harmonicseparator including a harmonic separation element having a firstBrewster angle surface, a total internal reflection surface, and asecond Brewster angle surface; and transmitting at least one of thefirst alternate wavelength light or the second alternate wavelengthlight from an interior portion of a laser frequency conversion system toa region external to the laser frequency conversion system.